The halogenated hydrocarbons CCl4 and CCl3Br are thought to be metabolized by the liver microsomal system via free radical intermediates. Attempts were made to trap the highly reactive intermediates with the aid of phenyl-t-butyl nitrone and 2-methyl-2- nitrosopropane. A stable free radical was trapped by phenyl-t-butyl nitrone when this compound was incubated with rat liver microsomes, CCl, Br and TPNH. Experiments with the other trapping agent were complicated by the light-induced formation of a radical species which made it difficult to detect any radicals during the metabolism of CCl3Br.